**3.3 Preparation of silica spheres**

Silica particles were prepared by modifying the Stöber method [22]. About 50 ml of ethanol, 2.5 ml of deionized water, and 4.25 ml of ammonium hydroxide were magnetically stirred in an 80 ml glass flask for 5 min. Then, 0.75 ml of TEOS was added dropwise. The solution was heated at 40°C. Temperature and agitation were kept for 2 h. The color of the solution changed from transparent to opaque white approximately 10 min after adding the TEOS as shown in **Figure 1**. This time corresponds to the induction period needed to form the SiO2 nucleus from the concentration used of the TEOS monomer [25]. Samples were obtained at 30, 60, 90, and 120 min after adding the TEOS to observe the evolution of the process.
